1. Field of the Invention
The present invention relates to an assembly including a metal casting and a polymeric member and, more particularly, such an assembly that is suitable for use in a transmission shift mechanism for a vehicle.
2. Description of the Related Art
Vehicles having an automatic transmission oftentimes have a shift lever mounted on the steering column of the vehicle. Many of these shift mechanisms have an actuator-driving crank arm that includes an arm portion component with a ball component mounted on the arm portion to provide the crank arm with a substantially spherically-shaped ball end. In some known mechanisms, the ball component of the crank arm is rotatable relative to its arm portion component while in other known mechanisms, the ball component is fixed to the arm portion and cannot rotate relative to the arm portion.
In such known mechanisms, the ball component is often disposed within an arcuate cam slot opening located on a pivoting actuator. Rotational movement of the crank arm moves the actuator between various positions about its pivot point. One end of a cable or linkage is attached to the actuator with the other end being attached to the gear selector of the transmission of the vehicle so that movement of the cable effects transmission gear selection.
The arm portion of the crank arm is typically steel, and requires machining to form the crank arm from extruded bar stock to control size tolerance and material consistency. The arm portion of such typical crank arms is generally manufactured from a steel rod in a progressive, cold forming stamping process that requires several separate forming and/or machining operations to yield the arm portion.
In a subsequent metal forming operation, the ball component of the crank arm is then attached, typically by being staked onto the stamping that forms the arm portion. Generally, the staking process will fix the ball on the crank arm in a manner that does not allow the ball to rotate relative to the crank arm. The assembly process whereby the ball component is assembled to the arm portion can produce scrap from over/under staking. Additionally, the staking of the ball sometime produces inconsistent positional tolerances and/or results in failure due to ball separation.
The arm portion requires selective orientation at the next level of assembly wherein the crank arm is attached to its rotatable shift shaft. The ball component is typically a self-lubricating plastic that has a shape formed by a machining process. When staking the ball component on the arm portion to non-rotationally fix the ball on the crank arm, the rotational position of the ball component is generally not controlled and, thus, the entire exterior surface of the ball must be precisely formed, even though in operation a substantial portion of the ball exterior surface may never engage a surface of the actuator cam opening.
The use of a staking process that fixes the ball component on the arm portion component without allowing the ball to rotate will generally be less expensive than attaching the ball component to the arm portion in a manner that allows the ball to rotate relative to the arm portion. For example, it is known to use a shoulder bolt to rotatably attach a ball component. Ball components used to manufacture crank arms using a staking process are generally formed by machining an extruded plastic material. By machining an extruded plastic material to form the ball, the dimensions of the ball can be controlled. Moreover, a ball formed by machining an extruded plastic material will have greater strength than a similar ball formed by injection molding. The differential cooling of an injection molded ball component will oftentimes reduce the strength of the ball near its exterior surface which, when non-rotatably staked to the arm portion, is in sliding engagement with the cam slot. Although the machining of extruded plastic material to form a ball component is generally more expensive than injection molding a ball component, injection molded ball components typically do not have the strength necessary to provide a durable and reliable product when placed in sliding engagement with the camming slot. Thus, the ball component must typically be formed using the relative more expensive process of machining an extruded material.
Accommodating the manufacturing and assembly-related factors of known shift mechanism crank arms make such crank arms undesirably expensive.